1. Field of the Invention
This invention relates to modular multiple channel fiber optic multiplexer, demultiplexer, multiplexer/demultiplexer, and/or add/drop components which employ Mach-Zehnder optical interferometers, and which may be used in dense wavelength division multiplexing (DWDM) communications systems or networks having channel spacings of less than one nanometer.
2. Discussion of Related Art
Fiber optic systems are presently being used for high bandwidth, high speed voice and video communications. Originally, single channel systems in which each fiber carried a single channel sufficed, but increases in traffic have led to a need for greater channel-carrying capacity. Because of the high cost of laying optical fibers, increasing capacity by laying more fibers is impractical, and thus achieving greater efficiency in utilizing fiber resources has become increasingly important, which has led to the development of systems for adding channels to existing fibers, and ultimately to the development of systems for providing two-way multiple channel communications over a single fiber.
In fiber optic systems, the most convenient way to carry multiple channels over a single fiber without interference between the channels, for both one-way and bi-directional communications, is by means of a technique known as wavelength division multiplexing, in which multiple signals of separate wavelengths are coupled to the fiber at the transmitting side and separated at the receiving side. Introduction of the different wavelengths into the fiber at the transmitting end and their removal at the receiving end can be accomplished in a number of different ways, including electronic multiplexing before conversion to a light signal, and by means of simple fused fiber couplers. Optical coupling is preferable to electronic multiplexing techniques because of its speed and simplicity, but maintaining adequate channel separation, rejecting noise, and minimizing losses are difficult to achieve with conventional optical couplers.
In order to improve channel separation and reduce noise and losses in the optical couplers used to combine or separate wavelengths for optical wavelength division multiplexer systems, and in particular dense wavelength division multiplexing systems, which are defined as systems having a wavelength or channel spacing of less than eight nanometers, it has recently been proposed to use a type of fiber optic coupler made up of a Mach-Zehnder interferometric filter to which Bragg gratings have been added, for the purpose of inserting or removing channels from a wavelength division multiplexed light signal. The Mach-Zehnder interferometric coupler is relatively simple in structure and yet provides greatly improved isolation of channels with relatively low loss in comparison with conventional fiber optic channel adding or removing couplers.
FIG. 1 shows a Mach-Zehnder interferometer 1 of the type which may be used in the claimed invention. A more detailed description of this type of interferometer may be found in U.S. Pat. No. 4,900,119, the disclosure of which is hereby incorporated by reference. The Mach-Zehnder interferometer 1 shown in FIG. 1 consists of two 50/50 fiber optic couplers 2 and 3 connected by two identical Bragg gratings 4 and 5. The first coupler has two ports 6 and 7, and the second coupler includes two ports 8 and 9, ports 6-9 being in the form of fiber ends which can be connected by splicing or any other convenient means to a signal source or receiver, or to other fibers.
When used for the purpose of removing a channel from a wavelength division multiplexed signal, the first port, port 6, serves as the input for the multiplexed signal from which the channel is to be removed, the input signal being equally split by the 50/50 coupler 2 and transmitted to the two identical Bragg gratings 4 and 5 which are arranged to reflect only the channel (or channels) to be removed, and to pass all other channels with minimal loss. The reflected channel or wavelength is then combined by the first 50/50 coupler 2 and output through the second port, port 7, while the channels which pass through the Bragg filters are combined by the second 50/50 coupler 3 and output through the fourth port, port 9. The third port, port 8, is terminated by conventional means and is generally not used, although it is possible to use port 8 to add back a signal at the same wavelength as the signal originally reflected by the Bragg grating and output through port 7.
When Mach-Zehnder interferometer 1 is used for the purpose of inserting a wavelength into a wavelength division multiplexed signal, the fourth port (port 9) is used as the input for the signal to which a channel is to be added, and the second port (port 7) is connected to the channel source, with Bragg gratings 4 and 5 again being arranged to reflect the channel being inserted and to pass other channels. In this case, 50/50 coupler 3 separates the signal input through port 9 into two equal parts which pass through filters 4 and 5 and are combined by 50/50 coupler 2 for output through port 6. The inserted channel is separated into two parts by 50/50 coupler 2, reflected by filters 4 and 5, and then re-combined in 50/50 coupler 2 for output through the first port (port 6) along with the signal originally input through port 9.
The Bragg gratings 4 and 5 used in the Mach-Zehnder interferometer 1 of FIG. 1 are illustrated in greater detail in FIG. 2, which shows an optical fiber 10 having a cladding 11 and core 12 within which the Bragg gratings are formed by irradiating germanium oxide (GeO.sub.2) sites in the core with interfering ultra-violet light beams, thereby creating permanent changes in the core in the form of repetitive refractive index changes which act as a physical grating. When light travelling down the core of the fiber, indicated by arrow A, encounters the grating, a portion of the light indicated by arrow B is reflected back and the remainder indicated by arrow C is passed with little loss.
FIGS. 3a-3c are idealized schematic representations of spectral plots respectively showing the incident light indicated by arrow A, the reflected light indicated by arrow B, and the light output indicated by arrow C for the Bragg grating arrangement of FIG. 2. A key feature of the spectrum shown in FIG. 3c is that a portion of the spectrum, on the short wavelength side of the notch formed by the Bragg grating is missing. The missing portion 13 is known as the short wavelength tail and represents losses in the Bragg grating. While losses also occur at higher wavelengths, the losses are markedly asymmetric and only the short wavelength losses have been depicted in FIG. 3c.
Mach-Zehnder interferometers of the type depicted in FIG. 1, utilizing the in-fiber Bragg gratings of FIG. 2, are now commercially available and form the basis for the fiber optic multiplexer/demultiplexer system of the present invention. However, although it has previously been proposed to use such interferometers for the purpose of inserting or removing channels in a wavelength division multiplexing system, the proposals have only generally indicated how the integration of the Mach-Zehnder interferometers into a practical fiber optic communication system are to be accomplished. In particular, while one way to integrate the interferometers would simply be to connectorize the three functional ports of the interferometer so that one interferometer and signal source and receiver could easily be connected to the main signal as needed, the use of connectors increases the overall signal losses in the device. On the other hand, fusion splicing of the interferometer ports would minimize losses but make component replacement and upgrading of the system impractical. In addition, any solution designed to minimize losses must address the short wavelength tail effect.
Thus, while Mach-Zehnder interferometer technology is ideally suited to the overall purpose of adding or removing channels in a wave-division multiplexing system, the use of wavelength division multiplexing permitting multiple channel uni-directional or bi-directional communications over single fibers without the need for laying additional fibers, a practical arrangement for incorporating the interferometers in a communication system with minimum signal loss, in which interferometers can easily be removed or added in the field for replacement or upgrading, and in which the interferometers can be retrofitted onto existing narrowband systems in order to increase the number of channels without having to replace existing components, has yet to be proposed. The present invention seeks to provide such a system.
It is noted that neither modularity or minimization of signal losses are new concepts. For example, a modular fiber optic wavelength division multiplexer/demultiplexer arrangement is disclosed in U.S. Pat. No. 5,557,439. However, this patent takes the approach of separately packaging each of the channel insertion or separation devices, and simply providing spare packages in the combined component in case the number of channels needs to be increased. Such an approach requires, in addition to the channel adding or removing packages or modules, a mixer on the multiplexer side and a splitter on the demultiplexer side for distributing the multiplexed signal to the individual channel separators and combining the outputs of the individual channel inserters, and while such an arrangement has the advantage of a purely parallel configuration in which each channel only passes through a single separator or inserter during multiplexing and demultiplexing, it is wasteful of resources. In contrast, the present invention uses a cascaded arrangement, in which the channel inserting or removing components are placed in series, eliminating the need for splitters, mixers, and multiple fiber connections, but amplifying the problem of signal losses.
Thus, a need still exists for a fiber optic multiplexer and demultiplexer components for use in multiple channel wavelength division multiplexing systems, having both acceptable signal losses and a high degree of modularity.